Hydrocarbon-water emulsions have been known for many years and have many uses, including that of fuel-water emulsions.
Such fuel-water emulsions have a number of advantages.
For example, in “NOx Reduction with EGR in a Diesel Engine Using Emulsified Fuel”, Y. Yoshimito et al., SAE Paper 982490, 1998, it is described how from environmental concerns reductions in NOx and particulate emissions from diesel engines had been mandated in recent years. It states that diesel engines using water-in-gas oil emulsified fuel have shown simultaneous improvements in NOx, smoke and fuel consumption.
In “Low Emission Water Blend Diesel Fuel”, D. T. Daly et al., Symposium on New Chemistry of Fuel Additives, 219th National Meeting, American Chemical Society, 2000, it is described that the addition of water to diesel fuel lowers emissions of particulates by serving as a diluent to the key combustion intermediates, and decreases NOx by lowering combustion temperatures through its high heat of evaporation.
In “AQUAZOLE™: An Original Emulsified Water-Diesel Fuel for Heavy-Duty Applications”, Barnaud et al., SAE Paper 2000-01-1861, 2000, it is described that the advantages of injecting water into an internal combustion engine included raising viscosity levels, removal of sediment, and reduction of nitrogen oxide emissions by reducing combustion temperature. There is also specific reference to reduction in black smoke and particulates emissions.
WO-A-99/13028 relates to emulsions comprising a Fischer-Tropsch derived liquid hydrocarbon, a non-ionic surfactant and water, and states that such emulsions are easier to prepare and more stable than the corresponding emulsions with petroleum derived hydrocarbons. There is specific reference to such emulsions having better emission characteristics than petroleum derived emulsions. However, WO-A-99/13028 is concerned with emulsions in which water is the continuous phase, i.e. oil-in-water emulsions.
WO-A-99/63025 relates to aqueous fuel compositions which exhibit reduced NOx and particulate emissions. It describes how the rates at which NOx are formed is related to the flame temperature during combustion in an engine. It describes how the flame temperature can be reduced by the use of aqueous fuels, i.e. incorporating both water and fuel into an emulsion. However, it indicates that problems that may occur from long-term use of aqueous fuels include precipitate deposition. It is described that water preferably functions as the continuous phase of the emulsion. Example 5 therein refers specifically to the test engine being modified to run a fuel-in-water emulsion. Therefore, although there is reference in said Example 5 to a fuel emulsion in which the diesel fuel was Fischer-Tropsch diesel, it is clearly a fuel-in-water emulsion. It also indicates that a significant barrier to the commercial use of aqueous fuel emulsions is emulsion stability.
As described in “The performance of Diesel Fuel manufactured by the Shell Middle Distillate Synthesis process”, Clark et al.,Proceedings of 2nd Int. Colloquium, “Fuels”, Tech, Akad. Esslingen, Ostfildern, Germany, 1999, the diesel cut from the SMDS process has very good cetane quality, low density, plus negligible sulphur and aromatics contents, such properties making it potentially valuable as a diesel fuel with lower emissions than conventional automotive gas oil (AGO). “The performance of Diesel fuel manufactured by Shell's GtL technology in the latest technology vehicles”, Clark et al., Proceedings of 3rd Int. Colloquium, “Fuels”, Tech, Akad. Esslingen, Ostfildern, Germany, 2001 describes SMDS diesel product and discusses the emissions benefits.
GB-A-2308383 describes water-in-oil emulsions in middle distillate fuel, particularly diesel fuel. It is directed to the reduction of emissions by the inclusion of an organic nitrate ignition improver.
Therefore, there are emissions advantages in using fuel-water emulsions. However, it is also known that ignition delay or lag is longer and cetane number is lower with emulsions based on conventional fuel than with non-emulsified conventional fuel.